Derby ('dər-bē) is a city in New Haven County, Connecticut, United States, approximately 8 miles west-northwest of New Haven. It is located in southwest Connecticut at the confluence of the Housatonic and Naugatuck Rivers. It borders the cities of Ansonia to the north and Shelton to the southwest, and the towns of Orange to the south, Seymour to the northwest, and Woodbridge to the east. The population was 12,903 at the 2010 census. It is the smallest city in Connecticut by area, at 5.3 square miles.
Derby was settled in 1642 as an Indian trading post under the name Paugasset. It was named after Derby, England, in 1675. It included what are now Ansonia, Seymour, Oxford, and parts of Beacon Falls.
Derby is home to the first electric trolley system in New England, only the second in the United States. It is also home to the first electric locomotive in U.S. history to be built and successfully used commercially for hauling freight. The locomotive, built in 1888, is still kept in running condition by the Shore Line Trolley Museum.
Derby was settled in 1642 as an Indian trading post under the name Paugasset by John Wakeman of New Haven, though fur traders had been in the area before and Native Americans had lived there for centuries.